This invention relates to strapping machines which fasten a steel or plastic strap around a load, and particularly to machines which operate upon palletized loads.
A great many manufacturing process result in the manufactured goods being put into cardboard cartons, the cartons being then stacked on pallets. To protect the goods during shipping or storage, the cartons are secured to the pallet by steel or plastic straps. Typically, the palletized load is strapped by a machine which passes a strap under the pallet, up the sides and across the top of the load. The strap ends are pulled taut, generally at the top of the load, and the ends of the strap are clamped together.
The strap must be sufficiently tautened so that shifting of the load on the pallet will not occur during shipment or handling by fork lift trucks. At the same time, the tension on the strap must not be so high as to cause a crushing of the cartons engaged by the strap, particularly the cartons on the upper tier around which the strap passes. Typically, a strapping machine will be set to tauten the strap at a relatively high tension but safely below the crush points of the cartons.
However, even though a tension is used which is below the crush point, oftentimes the strapping operation will result in a crushing of the upper edge cartons. As the strap is tautened it will be pulled across the upper edge of the load with increasing force. The friction resulting from this snubbing movement will increase the stress on the carton edge considerably beyond that which would be imposed by the strap tension if there were no movement of the storage across the edge.
In order to protect against such crushing, boards are often placed along the upper edge of the load between the strap and cartons to absorb the snubbing stress and prevent it from being applied to the carton edges. The boards are left in place until such time as the straps are removed and the cartons are unloaded. However, the labor and material involved results in a considerable increase in cost. Furthermore, the use of such boards increases the size of the load and decreases the number of pallets which may be stored or shipped in a given floor area.
It is a primary object of the present invention to provide an improved strapping machine wherein a dynamic snubbing stress will not be imposed on the load as it is being strapped so that the load will only be subjected to the static force of the finally tautened strap.